1. Field of the Invention
The present invention generally relates to a method of forming a semiconductor chip, solder balls, and an assembly structure.
2. Description of the Related Art
In respect of flip-chip assembly having a semiconductor chip mounted on a circuit board with its face down, an underfill technology is generally known that fills resin material between the semiconductor chip and the circuit board (see patent document 1, for example). When lead-free solder balls of large size are formed on a semiconductor chip, a photosensitive dry film resist may be used (see non-patent document 1, for example). Further, there is a technology that forms a large-scale solder ball free of lead by use of a resist mask (see non-patent document 2, for example).
Patent Document 1:
Japanese Patent Application Publication No. 9-172035
Non-Patent Document 1:
URL: Fujitsu Co., Ltd., “Forming Lead-Free Fine Solder Bumps at Half a Cost, [online], Dec. 12, 2001, [Searched on Sep. 3, 2002], Internet<URL:http://pr.fujitsu.com/jp/news/2001/12/1 2-1.html>
Non-Patent Document 2:
S. Sakuyama, et al., “Technology for Forming of Batch of Bumps on Semiconductor Wafer”, 7th Symposium Mate2001, Micro-Contact Research Committee, Feb. 1, 2001, p. 285-290
The underfill technology fills a gap between a semiconductor chip and a circuit board with thermosetting resin such as epoxy resin after the semiconductor chip and the circuit board are connected together by a flip-chip assembly technology. Heat is then applied to cure the thermosetting resin.
With the cured thermosetting resin filling the gap between the semiconductor chip and the circuit board, heat-induced deformation is reduced between the semiconductor chip and the circuit board. This improves the reliability of connections. Since the gap between the semiconductor chip and the circuit board is sealed by the thermosetting resin, corrosion can be prevented by keeping moisture away from solder balls, electrodes, etc.
FIG. 1 is an illustrative drawing showing a cross-sectional view of a semiconductor chip 10 and a circuit board 20 with a thermosetting resin 31 (underfill resin) provided between them by use of a related-art underfill technology. In this example, the semiconductor chip 10 having solder balls 22 formed on electrode pads 11 is mounted in the flip chip configuration on a wiring layer 21 of the circuit board 20 before filing the gap between the semiconductor chip 10 and the circuit board 20 with the thermosetting resin 31.
The thermosetting resin 31 is injected from the position shown by an arrow A into the gap formed between the semiconductor chip 10 and the circuit board 20. Heat is then applied to cure the thermosetting resin 31. With the provision of the thermosetting resin 31, heat-induced deformation between the semiconductor chip 10 and the circuit board 20 can be reduced, thereby improving the reliability of connections at the solder balls 22.
In the method of forming the thermosetting resin 31 as described above, the thermosetting resin 31 needs to be injected into a narrow gap that is formed between the semiconductor chip 10 and the circuit board 20 after the semiconductor chip 10 is mounted in the flip-chip configuration onto the circuit board 20. Filling every corner of the narrow gap with the thermosetting resin 31 is difficult, which may create areas (voids 41) where the thermosetting resin 31 is absent as shown in FIG. 1.
If a heating process for curing the thermosetting resin 31 is performed with these voids being in existence, moisture trapped inside the voids 41 expands due to heat, creating cracks in the thermosetting resin 31 or resulting in severance of the solder balls 22 from the wiring layer 21.
Accordingly, there is a need for a semiconductor chip, a method of forming solder balls, and an assembly structure that offer superior property in terms of the strength of contacts and the reliability of electrical connections of the assembly structure.